Tools
Junior ISA Calculator UK: What Your Child Gets at 18
A Junior ISA lets you put up to £9,000 a year into a tax-free account your child cannot touch until 18. The length of that runway is the whole point: money paid in at birth has 18 years to compound, money paid in at 15 has three, so the same monthly sum can end up worth very different amounts depending on when you start. This calculator shows the pot at 18, how much of it is growth, what it is worth in today's money, and how much you would need to pay in each month to reach a figure you have in mind.
Work out your child's Junior ISA at 18
How the calculator works
It counts the months left until the child's 18th birthday and runs the account forward one month at a time. Your monthly payment goes in at the start of each month, family gifts go in once a year starting now, and the whole pot grows at the monthly equivalent of the yearly rate you enter. Anything you already hold, including a Child Trust Fund you have moved across, grows from today.
The growth figure should be after all charges: the fund's ongoing charge and the platform fee. On a stocks and shares JISA, a 0.5% difference in yearly charges compounds over 18 years in exactly the way growth does, just in the wrong direction. Our platform fee calculator shows what your provider's fees cost in pounds.
The today's money figure divides the pot by the inflation you enter, compounded over the same period. It answers the more useful question: what will this buy when your child turns 18, measured in prices you recognise now. A target you enter is treated the same way, so asking for £30,000 means £30,000 of today's spending power, and the calculator raises the cash figure it aims for to allow for inflation.
The £9,000 limit
The Junior ISA allowance for 2026/27 is £9,000 per child. It covers everything paid in by anyone, across a cash and a stocks and shares JISA combined, and unused allowance cannot be carried forward. The calculator warns you if your monthly payment plus family gifts goes over £9,000 in a year, or if the monthly sum needed to reach your target would. It treats each 12 months from today as a year, which is close to, but not the same as, the tax year that runs from 6 April.
The allowance is separate from your own £20,000 adult ISA allowance, so paying into your child's account does not reduce what you can shelter for yourself. The full rules, including who can open an account and what changes at 16, are in our Junior ISA guide.
What happens at 18
On the 18th birthday the Junior ISA becomes an adult ISA in the child's name and the money is theirs to spend as they choose. Nobody can withdraw it before then, apart from in cases of terminal illness or death. That lock is what makes a stocks and shares JISA reasonable for a baby: the money cannot be pulled out in a bad year. As 18 gets closer, some parents move part of the pot into cash so a market fall in the last year or two does not dent the final figure.
Related
- Junior ISAs explained: allowance, types and what happens at 18
- Cash ISA vs stocks and shares ISA
- How to transfer an ISA, including between JISA providers
- Investment growth calculator for your own savings
Projections are estimates, not promises. Investment values fall as well as rise, and rules and limits can change. This is information, not financial advice.
